CuInSe2 and its related alloys, including CuInS2, CuGaSe2, CuGaS2, Cu(InxGa1-x)Se2, Cu(InxGa1-x)S2, and Cu(InxGa1-x)(SySe2-y) where 0≦x≦1 and 2≦y≦0, (collectively known as CIGS), are some of the most promising candidates for photovoltaic applications due to their unique structural and electrical properties. CIGS thin film solar cells are highly stable against radiation, which makes them ideal for space applications. High efficiency solar cells have been fabricated based on CIGS absorber films grown by various techniques, with the highest reported efficiency of 19.2% reported using vacuum co-evaporation.(1) The highest quality CIGS thin films have been traditionally fabricated using vacuum co-evaporation, however, the resulting production costs of such fabrication processes are typically high, thereby limiting its usefulness in large-scale mass production applications. There are also issues associated with uniformity of the film for roll-to-roll processing. Thus, there has been a continuing effort to develop thin film deposition techniques for large area substrates using cost effective techniques.
To circumvent the limitations of vacuum co-evaporation, several techniques based on the selenization of metallic or binary precursor layers and particulate precursor films have been reported.(2-7) The selenization of pre-deposited Cu/In metal precursors on substrates by either H2Se gas or Se vapor is currently favored for commercial manufacturing processes. However, these selenization processes are complicated and timely, as well as require high operating temperatures, thereby resulting in increased processing costs and low production rates. In addition, the required use of highly toxic gases during the selenization process (such as H2Se, for instance), as well as the use of high-end equipment to safely maintain the increased temperature levels, significantly adds to the fabrication costs associated with these processes. Although there are several commercialized processes based on the selenization of precursor films, the inherent drawbacks associated with these processes (e.g., high costs, composition control and material utilization issues) limit the mass utilization of the CIGS photovoltaic cells.
Other techniques, including electrodeposition,(8) chemical vapor deposition,(9-11) and spray deposition,(12, 13) have also been explored for the fabrication of CIGS thin films. These techniques, however, are limited due to low material utilization, as well as low crystallinity and small crystalline sizes of the as-synthesized thin films.
Recently, nanocrystalline semiconductors have attracted a considerable amount of attention due to their unique physiochemical properties and potential applications in novel optical, electrical, and optoelectrical devices. Several groups have demonstrated the use of nanoparticle building blocks for the fabrication of nanostructured solar cells. For instance, Gur et al. demonstrated the fabrication of air stable inorganic solar cells by spin coating thin films of CdTe and CdSe nanoparticles.(14) Also, previous work on the fabrication of CuInSe2 (CIS) and Cu(InGa)Se2 (CIGS) thin films using amorphous Cu—In—Se and Cu—In—Ga—Se nanoparticles, respectively, has been reported by Schulz et al.(15) However, the nanoparticles employed in this study were amorphous and high temperature annealing under a selenium environment was required to achieve the desired crystalline structure. Thus, CIS and/or CIGS nanoparticles having the desired composition and crystalline structures are expected to be ideal candidates for low cost solar cells, particularly as they allow the use of low-cost coating techniques, such as spray printing, spin coating, and doctor blading. In addition, by using CIS or CIGS nanoparticles with fixed compositions and crystalline structures, the use of high temperature selenization processes under toxic H2Se gas can be minimized or even eliminated. Furthermore, the composition of the film could be easily controlled on all scales by controlling the composition of the nanoparticles. Composition uniformity allows relatively large tolerance in the thickness of the film, such that traditional coating techniques (such as spin coating, dip coating, and spray printing) can be employed to fabricate CIS or CIGS thin films. All of these advantages will significantly simplify the manufacturing process and lower the fabrication cost of photovoltaic devices.
Consequently, a simple, controlled, and tunable process for the synthesis of CIS or CIGS nanoparticles with the right composition and crystalline structure needs to be developed. Several techniques have previously been reported in the literature for the synthesis of CuInSe2 and related nanoparticles. For instance, Carmalt et al. presented the solid state and solution phase metathesis synthesis of copper indium chalcogenides using metal halides and sodium chalcogenides as precursor materials.(16) For the solid-state metathesis reaction, the reaction was conducted inside a sealed ampoule and heated to 500° C. for 48 hours to produce single-phase CuInSe2 particles. Although solid-state metathesis has been utilized for the synthesis of binary materials, it is difficult to use for the synthesis of ternary or multinary materials due to possible phase segregations. Thus, solid-state metathesis typically requires an extensive period of time to ensure the formation of a ternary phase. The solution phase metathesis reaction has also been presented by Carmalt et al. where the same precursors were refluxed in toluene for 72 hours. The solution phase metathesis reaction allows the use of low temperature synthesis; however, the particles produced are amorphous and require high temperature annealing at 500° C. for 24 hours in order to obtain the desired crystalline structure.
Similar solution phase metathesis reactions have also been employed by Schulz et al. in their synthesis of CIGS nanoparticles.(15) In this case, the Cu—In—Ga—Se nanoparticles were prepared by reacting a mixture of CuI, InI3, and GaI3 in pyridine with Na2Se in methanol at a reduced temperature and under inert conditions. The nanoparticles produced in this reaction, however, were also amorphous and high temperature annealing was required to achieve the desired crystalline material.
Another method (“hot injection method”) was pioneered by Murray et al. to synthesize various metal and semiconductor nanocrystals, particularly those having diverse compositions, sizes and shapes.(17) In a typical ‘hot injection’ synthesis, organic ligands are used to passivate the surface of the nanoparticles to prevent particle aggregation. Moreover, nanoparticles with monodispersed sizes and shapes can be synthesized by controlling the concentration and functional group of the organic ligands.
The synthesis of CuInSe2 nanoparticles using the “hot injection technique” was first presented by Malik et al. in trioctylphosphine oxide (TOPO) and trioctylphosphine (TOP) by a two step reaction.(18) In this reaction, a TOP solution of CuCl and InCl3 was injected into TOPO at 100° C. and then followed by a hot injection of trioctylphosphine selenide (TOPSe) at an elevated temperature of 330° C. to initiate the nucleation and growth of nanoparticles. Spherical CuInSe2 nanoparticles of about 4.5 nm were synthesized according to the authors, and the Powder X-Ray Diffraction (“PXRD”) data presented indicated that binary materials such as Cu2Se and In2O3 were present as by-products.
In another study relating to the pyrolysis of molecular single source precursors, the stoichiometry precursor (PPh3)2CuIn(SePh)4 was used in the synthesis of a CuInSe2 nanoparticle using spray pyrolysis.(19) While nanocrystalline CuInSe2 particles ranging from about 3-30 nm were produced by the thermal decomposition of the molecular precursor, the synthesized nanocrystals typically agglomerated into large clusters. Moreover, the PXRD data indicated that CuInSe2 nanocrystals were only produced at high temperatures (e.g., from about 275° C. to about 300° C.). However, no direct images of the nanocrystals were presented. Some of the drawbacks of this process are that the preparation of the molecular precursors could be difficult and costly, as well as require low material utilization.
More recently, Grisaru et al. presented a microwave-assisted synthesis process of CuInSe2 nanoparticles using CuCl and elemental In and Se as precursors in ethylene glycol based solvents.(20) While the reaction time was much faster applying microwave heating, the synthesized nanoparticles lacked defined shapes and sizes, and generally agglomerated together into large clusters. Furthermore, small amounts of Cu2Se were also detected as by-products from the reaction as shown from the PXRD data presented by the authors.
Another process, which was presented by Li et al., involved the preparation of CuInSe2 nanowhiskers and nanoparticles using CuCl2, InCl3, and Se as reagents in ethylenediamine and diethylamine, respectively, and particularly using a solvothermal route.(21) It was suggested by the authors that amine served as a structure directing agent in the solvothermal synthesis. PXRD characterization of the nanoparticles showed a clean single phase of chalcopyrite CuInSe2. Jiang et al. also explored the solvothermal synthesis of CuInSe2 nanorods and nanoparticles using elemental Cu, In, and Se.(22) Chun Y G et al. further expanded the synthesis into quaternary Cu(InGa)Se2 nanoparticles by solvothermal reaction of elemental Cu, In, Ga, and Se in ethylenediamine.(23) However, generally the nanoparticles synthesized using solvothermal techniques were highly polydispersed. A key feature of these solvothermal syntheses is that they are conducted in a closed autoclave and generally require from about 15 hours to a few days to perform. The reaction is also conducted at pressures much higher than atmospheric pressures and requires pressurized equipment because of the low normal boiling temperatures of the solvents used during the synthesis. For example, the normal boiling temperature for ethylenediamine and diethylamine is about 118° C. and 55° C., respectively. FIG. 1 shows that ethylenediamine and diethylamine have very high vapor pressures over the range of reaction temperatures usually used in the solvothermal synthesis. Such equipment and associated handling procedures add cost to the final product and are less amenable to very large-scale production such as is typically needed for world-scale solar panel manufacturing plants.
Although several methods have been reported on the synthesis of CuInSe2 nanoparticles, none of the above-mentioned techniques are able to sufficiently control the size, shape, crystallinity and/or purity of the nanoparticles. Furthermore, many of the above-mentioned techniques typically require long reaction times. Thus, it is desirable to develop a fast and efficient process capable of producing crystalline CIS or CIGS nanoparticles without resulting impurities or by-products. As such, the present teachings are intended to overcome and improve upon these and/or other shortcomings currently found within the prior art.